This invention relates to vehicle windshield wipers having a finely tuneable intermittent operating mode. More particularly the invention relates to an apparatus and method for improving the ability of an operator of a vehicle to set the amount of the dwell (pause) in an intermittent wiper system to a precisely desired time.
Contemporary intermittent windshield wipers set dwell times through the use of one or more switches and either a continuously variable resistor (potentiometer) or discretely selected resistors, arranged in an RC (resistor-capacitor) timing circuit. A major problem with the potentiometer arrangement is that an instant wipe cannot be obtained except by decreasing the circuit resistance at a time near the end of a dwell. Another disadvantage of the potentiometer type of wiper control is its non-linear operation. This makes it difficult to fine tune the dwell time, especially for shorter durations.
The discretely selected resistor switch (typically 5 to 7 detented positions) overcomes both of these disadvantages by using resistors of exactly known resistance for each time setting and arranging them between discrete contacts points. The problem with that arrangement is that relatively few distinct settings are available. This severely limits the ability of the system to satisfy operator preferences.
Wiper control systems which rely upon RC time constants for timing control are susceptible to timing errors caused by component tolerances, variations in triggering thresholds, and temperature induced resistance changes. Another problem is that very large resistance values are necessary in order to accommodate long dwell times (greater than about 10 seconds). A general teaching of intermittent wiping control based upon RC time constants may be found in Tracht U.S. Pat. No. 4,825,134 and references mentioned therein.
Graham U.S. Pat. No. 4,492,904 discloses a technique for overcoming some of the problems of RC timing circuits. Graham teaches a "one touch" wiper having a single switch for setting an intermittent wiping dwell time. When the switch is depressed an instant wipe is obtained and a digital timing circuit begins to run. If the switch is not depressed again, the timer times out, and the system self sets the dwell time to a predetermined maximum duration of approximately twenty seconds. If the switch is depressed a second time before the time out, the timer value is stored as a desired dwell time.
A problem with single-switch dwell period adjustment is that the technique is quite foreign to the average driver. Drivers of vehicles equipped with such wiping control systems have had difficulty acclimating themselves to the procedure. As, a consequence, systems of this type have been plagued by operator-induced problems which erroneously have been attributed to equipment malfunction. Studies have shown that drivers prefer to adjust their wiper dwell periods by more familiar methods such as rotating a column switch or linearly sliding a friction switch to a selected one of a series of positions.